1. Field of the Invention
The present invention relates to an optical information recording and/or reproducing apparatus for recording and/or reproducing information in such a way as to ra diate a plurality of beams onto an optical information recording medium having information tracks and tracking tracks and to receive reflected light therefrom by a photodetector.
2. Related Back ground Art
A variety of forms including a disk, a card, a tape, and so on are conventionally known for information recording media for recording and/or reproducing information with light. These optical information recording media include those capable of recording and reproducing information, those capable of only reproducing information, and so on. Information is recorded in a recordable medium in the form of an optically detectable information bit string while scanning an information track with an optical beam modulated according to recording information and focused in a micro spot.
The information is reproduced from the recording medium by scanning the information bit string in the information track by a light beam spot of constant power weak enough to avoid recording into the recording medium (or too weak to make the information bit string) and detecting reflected light or transmitted light from the recording medium.
An optical head used for recording and/or reproduction of information into or from the recording medium as discussed above is arranged to be movable relative to the recording medium in a direction of the information tracks and in a direction traversing the information track direction, and movement of the optical head causes the light beam spot to scan an information track. A focusing lens for focusing the light beam spot in the optical head is, for example, an objective lens. This objective lens is held on the optical head body so as to be movable independently in the axial direction (or in the focusing direction) and in the direction perpendicular to both the axial direction and the information track direction of the recording medium (or in the tracking direction). Such holding of the objective lens is normally effected through an elastic member, and the movement of the objective lens in the above two directions is usually effected by drive of an actuator utilizing magnetic interaction.
A card type recording medium (hereinafter referred to as an optical card) as shown in FIG. 1 is known as an optical information recording medium of this type. Here, many information tracks 2 are arranged parallel to the L-F direction on an information recording surface of the optical card 1. Also, a home position 3 to be a reference position in access to the above information tracks 2 is provided on the information recording surface of the optical card 1. The information tracks 2 are arranged, for example, in the order of reference symbols 2-1, 2-2, 2-3, . . . from the side near the hom e position 3. Further, tracking tracks 4 are provided adjacent t o these information tracks in the order of reference symbols 4-1, 4-2, 4-3, . . . , as shown in FIG. 2. These tracking tracks 4 function as a guide for autotracking (hereinafter referred to as AT) to control the beam spot so as not to depart from a predetermined information track during scanning of the light beam spot for recording or reproducing information.
This AT servo is carried out in such a way that, in the optical head, a deviation (AT error) of the above light beam spot from the information track is detected, a detection signal thereof is negatively fed back to the tracking actuator, and the objective lens is moved in the tracking direction (in the direction D) relative to the optical head body, whereby the light beam spot can be made to follow the desired information track.
In addition, autofocusing (hereinafter referred to as AF) servo is carried out in order to shape (or focus) the light beam in a spot of an appropriate size on the surface of the optical card during scanning of the information track with the light beam spot upon recording or reproduction of information. This AF servo is carried out in such a way that, in the optical head, a deviation (AF error) of the light beam spot from an in-focus state is detected, a detection signal thereof is negatively fed back to the focusing actuator, and the objective lens 28 is moved in the focusing direction relative to the optical head body, whereby the light beam spot can be focused on the surface of the optical card.
In FIG. 2, S1, S2, S3 represent light spots on the optical card, among which the light spots S1, S3 are used for the AT control and the light spot S2 for the AF control, production of information bits upon recording, and reading of information bits upon reproduction. In each information track, 6-1, 6-2 and 7-1, 7-2 indicate a pre-formatted left address portion and a right address portion, respectively, and an information track can be identified by reading this address portion. In the drawing, reference symbols 5-1 and 5-2 denote data portions (information bits).
In FIG. 3, numeral 21 designates a semiconductor laser as a light source, which emits, in this example, light having a wavelength of 830 nm polarized in the normal direction, onto the tracks. Numeral 22 designates a collimator lens, 23 a beam shaping prism, 25 a diffraction grating for splitting a light beam, 25' a diffraction grating portion, and 26 a polarizing beam splitter. Further, numeral 27 denotes a quarter wave plate, 28 an objective lens as a converging optical system, 29 a spherical lens, 30 a cylindrical lens, and 31 a photodetector. This photodetector 31 is composed of two light receiving elements 31a, 31c and a quartered light receiving element 31b, as shown in FIG. 4.
A light beam emitted from the semiconductor laser 21 is incident in the form of an elliptical divergent beam to the collimator lens 22. This lens collimates the light beam into a parallel beam, and the beam shaping prism 23 further shapes the parallel beam into a beam having a predetermined light intensity distribution, i.e., a circular intensity distribution (Gaussian distribution).
After that, the beam is incident to the diffraction grating 25, and the diffraction grating portion 25' having a smaller diameter than the diameter of the incident beam splits the incident beam into three light beams (the zeroth-order diffracted light and .+-. first-order diffracted light). These three beams are different in diameter for respective purposes. In this case, the zeroth-order diffracted light consists of diffracted light and non-diffracted light, and the beam diameter of the zeroth-order diffracted light is dominated by the beam diameter of the non-diffracted light. Naturally, the beam diameter of the zeroth-order diffracted light is larger than the beam diameter of the .+-. first-order diffracted light including only diffracted light. In the ordinary construction, beams of the zeroth-order diffracted light and .+-. first-order diffracted light are produced in the same beam diameter from the diffracted light and are used for the 3-beam AT method. The method for differentiating the beam diameter of the zeroth-order diffracted light from that of the .+-. first-order diffracted light, as described above, has been schemed out in order to realize higher accuracy of the AT control, to further enhance the recording density, or to raise the recording speed. The 3-beam AT method is based on such control as to keep a difference output zero between the output from the light receiving element 31a and the output from the light receiving element 31c. The three beams, thus split by the diffraction grating 25, are incident as beams of p-polarized light to the polarizing beam splitter 26.
Then, the three beams, as passing through the quarter wave plate 27, are converted into beams of circularly polarized light, and they are focused on the optical card 1 by the objective lens 28. The focused beams are three micro beam spots S1 (the + first-order diffracted light), S2 (the zeroth-order diffracted light), and S3 (the--first-order diffracted light), as shown in FIG. 2. Spot S2 is used for recording, reproduction, and AF control while spots S1 and S3 are used for AT control. Each spot is located on the optical card so that, as shown in FIG. 2, the light spots S1 and S3 are located on adjacent tracking tracks 4 while the light spot S2 is located on an information track 2 between the tracking tracks.
The reflected light from the light spots thus formed on the optical card 1 again passes through the objective lens 28 to be collimated, and the collimated beams pass through the quarter wave plate 27 to be converted into beams with the direction of polarization rotated 90.degree. from that upon incidence. They are incident as beams of s-polarized light to the polarizing beam splitter 26 to be reflected and guided to a detection optical system.
The detection optical system has a combination of the spherical lens 29 with the cylindrical lens 30, and the AF control is carried out according to the astigmatic method with the combination. In the astigmatic AF method, such control is made as to keep a difference output zero between two sums of outputs from respective diagonal segments of the quartered light receiving element 31b in the photodetector 31.
The three beams reflected from the optical card 1 each are converged by the detection optical system to be incident to the photodetector 31 and to form three light spots thereon. The light receiving elements 31a, 31c receive the reflected light of the foregoing light spots S1, S3, and the AT control is carried out using a difference between outputs from these two light receiving elements. Further, the quartered light receiving element 31b receives the reflected light of the light spot S2, and the AF control is carried out using the outputs therefrom. In addition, reproduction of recording information is also carried out using the outputs from the light receiving element 31b.
When the whole optical system of the optical head as described above is moved relative to the recording medium, the light beam spot S2 can scan the information track.
If the optical head is of a separate type, as shown in FIG. 7, consisting of a fixed head 40 fixed to a part of the apparatus and a movable head 41 movable in directions along the optical axis extending from the fixed head (in directions along the arrow Y in the drawing) relative to the fixed head 40, the fixed head 40 consists of an irradiation optical system of from the light source 21 to the shaping prism 23 and the detection optical system of from the spherical lens 29 to the photodetector 31, while the movable head 41 consists of the diffraction grating 25 for diffracting a part of the beam from the irradiation optical system of the fixed head and the converging optical system of from the polarizing beam splitter 26 to the objective lens 28. The movable head translationally scans the beam from the fixed head 40 relative to the information track in the optical-axis directions (in the directions of arrow Y in the drawing), while the optical card 1 is moved in the directions of arrow X in the drawing, whereby the entire surface of the optical card can be scanned in order to record and reproduce information. In the drawing, reference numeral 43 denotes a photodetector for monitoring a quantity of emitted light and 44 a bending mirror for changing the direction of a beam.
The two conventional examples, however, had the following problems. Namely, it is very difficult in the two conventional examples to align the axial center of the beam from the light source 21 with the center of the diffraction grating portion 25' because of a positional adjustment error of the light source 21, a position error of the diffraction grating 25 in the direction perpendicular to the optical axis, or a fabrication error of the diffraction grating portion 25'. Especially, in the latter conventional example, when the moving direction of the movable head 41 has a relative inclination to the axis of the emergent beam from the irradiation optical system of the fixed head 40, the movement of the movable head 41 will shift the position of incidence of the light to the diffraction grating 25 disposed at the position of incidence of the light, within the surface thereof. This means that deviation occurs between the center of the diffraction grating portion 25' of the diffraction grating 25 and the center of the incident beam. Further, the two conventional examples have no specific means for aligning the centers with each other.
This failure in aligning the centers will result in a drawback that a light intensity distribution of each of the zeroth-order diffracted light and .+-. first-order diffracted light emerging from the diffraction grating 25 is not axially symmetric with respect to the center of a beam.
As shown in FIG. 5, the center axis 53 (the triangle in the drawing) of the diffraction grating portion 25' of the diffraction grating 25 is shifted from the center axis 52 (the cross in the drawing) of the incident beam 51 incident to the diffraction grating 25. In such a state, the light intensity distributions of the three diffracted light beams from the diffraction grating 25 are given as shown in FIGS. 6A and 6B. FIG. 6A shows the light intensity distribution of the .+-. first-order diffracted light while FIG. 6B shows the light intensity distribution of the zeroth-order diffracted light. As seen also from these light intensity distributions herein, they are asymmetric with respect to the center axis of each beam.
Two big problems occur when the symmetry of the light intensity distribution of the beam is lost. One of them is an AT offset. As shown in FIG. 2, the light spots S1 and S3 for AT each are positioned inside the track (though they may be located outside) with respect to the light spot S2 for AF and reproduction. Since the light spots S1 and S3 are the .+-. first-order diffracted light, they have the same light intensity distribution. They are incident onto the tracking tracks in the asymmetric light intensity distribution shown in FIG. 6A. When the AT control (the 3-beam AT method) is carried out so as to equalize the outputs from the light receiving elements 31a and 31c of the photodetector 31 shown in FIG. 4 or so as to keep the output difference zero between the outputs from the two light receiving elements, as described previously, an offset inevitably occurs, thus causing positional deviation of a recording information or degradation of reproduction signal.
The other problem is an AF offset. The AF control is a control to keep the difference zero between sums of diagonal segments of the quartered light receiving element 31b shown in FIG. 4 (in the astigmatic AF method). Position adjustment of the quartered light receiving element is carried out by moving the sensor lens system (the detection optical system) in the optical-axis direction so that the difference is kept zero between the sums of outputs from the diagonal segments, for the incident light having the asymmetric light intensity distribution of FIG. 6B. Since the light intensity distribution is originally asymmetric, a state of the difference being zero will result in locating the sensor lens system at a position shifted from the regular position of the sensor lens system.
As a result, a control signal for the astigmatic AF control, the so-called S-shaped signal, will lose a balance of its waveform, which makes AF easier to be off by internal or external vibration. This raises the problem that a stable recording and/or reproducing operation is not assured.